Materials having the property of abradability are commonly used in numerous applications, and in particular for forming seals. Abradable seals are used in particular in association with the rotary parts of a turbomachine, such as its compressors, in order to reduce leakage of air or gas that might otherwise affect the efficiency of the turbomachine.
Such a turbomachine compressor consists in a plurality of blades secured to a shaft which is mounted in a stationary ring. In operation, the shaft rotates together with the blades inside the compressor ring.
In order to guarantee suitable efficiency for the turbomachine, it is important for leaks of air and gas in the compression sections of the turbomachine to be reduced to as little as possible. This reduction of leaks is obtained by minimizing the clearance that exists firstly between the tips of the blades and the inside surface of the compressor ring, and secondly between the inter-disk shrouds and the outside surface of the stator. Nevertheless, thermal expansion and centrifugal expansion of the compressor blades makes it difficult to obtain small clearance between the tips of the blades and the inside surface of the compressor ring.
Under such conditions, the inside surface of the compressor ring is generally covered in a coating of abradable material, and the shaft of the compressor is mounted in the compressor ring in such a manner that the tips of the blades are as close as possible to the abradable coating. The role of such an abradable coating is thus to form a seal between the stationary portions and the moving portions of the compressors of a turbomachine.
When contact occurs between the stationary portions and the moving portions of the compressor, the seal of abradable material makes it possible to obtain small clearance without damaging the parts of the rotor that come into contact. Interference between the stationary parts and the moving parts of compressors is due essentially to differential expansion of the stationary and moving parts during transient conditions in the operation of such compressors. Phenomena of blade creep, unbalance, and vibration can also lead to such interference.
In such interference situations, it is important for the seals to satisfy the following criteria:                the tips of the blades must not be subjected to excessive wear. Although a small amount of wear can be tolerated, it is preferable in the event of contact for it to be the seal that is damaged;        contact between the blade tips and the gaskets must not lead to the blades being heated, particularly when the blades are made of titanium alloy, where such heating can start a fire;        the seals must withstand the erosion caused by the flow of gas traveling in the flow section of the compressor;        the seals must also conserve their abradability in an environment that is oxidizing and corrosive. The rise in temperature in compressors leads to oxidation and the combustion gases of the turbomachine and the outside air lead to corrosion of the environment;        in the event of the seals being worn, the residue must not obstruct the orifices for cooling the compressors;        finally, the abradable material forming the seals must withstand high temperatures without presenting modifications such as hardening, becoming brittle, and crumbling, which could degrade its capacity to be abraded. The abradable material must be capable of withstanding the various operating cycles of the turbomachine without being degraded.        
Numerous powder materials for forming abradable seals have been proposed. These various materials can be classified mainly into two categories: materials having a silicon-based metal powder (e.g. a material comprising an AlSi alloy and an organic powder); and materials having a metal powder based on chromium and nickel (e.g. a material containing an NiCrAl alloy and a ceramic, organic, or clay powder). Nevertheless, those abradable materials suffer from drawbacks depending on the category to which they belong.
Materials based on silicon possess abradability and erosion characteristics that are satisfactory, but their suitability for use at high temperatures is limited. For example, the powder material described in U.S. Pat. No. 5,434,210 is known. That material is limited to a utilization temperature of about 400° C. Above that temperature, the metal matrix of the material shrinks and densifies, which can lead to wear on the facing blade tips.
As for materials based on chromium and nickel, they are relatively stable and good at withstanding high temperatures, however their abradability and erosion characteristics are not good enough, particularly when they are deposited facing compressor blades made of non-coated titanium alloy. For example although an NiCrAl alloy has good high-temperature behavior, it is relatively-hard, and thus leads to excessive wear of the blades.
In order to mitigate such a drawback, it is possible to have recourse to a protective coating on the tips of the blades. Nevertheless, the-use of such a coating is particularly expensive.